Almost 25% of the US population suffers from non-alcoholic fatty liver disease (NAFLD) that can progress to steatohepatitis (NASH), fibrosis, cirrhosis and hepatocellular carcinoma (HCC). Our recent studies, as well as others, link endoplasmic reticulum stress (ER stress) with non-alcoholic fatty liver disease (NAFLD). However, it is unknown whether/how ER stress impacts NASH development and/or HCC initiation and progression. In collaboration with Dr. Michael Karin (UCSD), our exciting preliminary data demonstrate that induction of ER stress in hepatocytes by excessive transgene expression of urokinase or a misfolded protein combined with a high fat diet (HFD) activates the unfolded protein response (UPR) and is sufficient to cause symptoms similar to human NASH with progression to HCC. This fundamental finding provides the basis for our proposed studies to identify the role of ER stress and cell death signaling in cell transformation and in the tumor microenvironment through the use of murine models harboring conditional gene deletion of selective UPR signaling components. Although many studies have focused on tumor progression, ours is unique because we will identify factors that initiate tumorigenesis. To elucidate the role of the UPR in cancer initiation we will test four hypotheses through our specific aims (SA). SA1: ER stress can initiate NASH and HCC formation. We will test whether induction of ER stress contributes to NASH and HCC development using two independent models of ER stress. We will also test whether restoration of ER homeostasis decreases tumor formation. Furthermore, in collaboration with Dr. Peter Metrakos (McGill University), we will be able to verify the findings from mouse model in human patients. SA2: Elimination of the proapoptotic arm of the UPR (PERK/eIF2?/ATF4/CHOP) will promote NASH and HCC initiation. We will eliminate eIF2? phosphorylation, ATF4 or CHOP in hepatocytes to determine their requirement in HCC development, and test whether the effect increased or decreased ATF4 and/or CHOP in HCC development is cell autonomous by inoculation of HCC progenitors into MUP-uPA mice fed with HFD. SA3: NASH/HCC development is restrained by the PERK/eIF2?/ATF4/CHOP pathway in the microenvironment. ATF4 and CHOP will be specifically deleted and eIF2? phosphorylation eliminated in stromal cells to test their requirement for HCC progression and determine their impact on inflammation and angiogenesis. Although extensive efforts are being directed to target UPR signaling in cancer, it remains unknown whether the UPR promotes or limits tumor initiation and/or development. Our proposed studies will provide essential key unprecedented insight into these questions that should impact efforts to target the UPR in cancer.